1. Field of the Invention
The invention relates to apparatus for fabricating thin films of complex compounds that form part of electrical components in integrated circuits, and more particularly, apparatus for forming such thin films from misted liquid precursors.
2. Description of the Related Art
As is well-known in the art, the electrical components in integrated circuits are made up of layers of thin films which are connected by wiring layers and separated by insulating layers. While some of the simple materials and compounds, such as silicon glass, have been formed using a liquid deposition process, thin films of complex compounds, that is, compounds containing more than two elements, in the prior art have always been formed using processes such as vacuum sputtering, (i.e., E-beam, D.C., R.F., ion-beam, etc.); laser ablation, reactive chemical vapor deposition, including metalorganic chemical vapor deposition (MOCVD); and liquid application methods using sol-gels (alkoxides) or carboxylates. However, none of these known methods have been able to produce metal oxides with properties good enough for use in integrated circuits. In all of the prior art processes, except sputtering, the films produced had significant physical defects, such as cracking, peeling, etc. It was impossible with the prior art processes, particularly sputtering, to reliably and repeatably produce metal oxides with a specific stoichiometry within tolerances required for integrated circuits. Some processes, like CVD, could be dangerous or toxic. Most required high temperatures that were destructive to an integrated circuit, and provided poor "step coverage" of a substrate to be covered; i.e., the prior art techniques resulted in a relatively excessive build-up of deposition of the film at the boundary of any discontinuities on the substrate. In prior art liquid deposition processes, it was impossible to control thickness with the degree of accuracy that is required to manufacture integrated circuits. As a result, up to now, metal oxides and other complex materials have not been used extensively in integrated circuits except for one or two specialty, relatively expensive applications, such as the use of sputtered PZT in ferroelectric integrated circuits that were expected to have short life times.
Ultraviolet radiation has been known to be effective in assisting chemical reactions in forming thin films. For example, see Japanese Patent Application No. 61183921 of Kamei, Japanese Patent Publication No. 60-128264 (NEC Corp), U.S. Pat. No. 4,811,684 issued to Tashiro et al., U.S. Pat. No. 4,683,147 issued to Eguchi et al., Japanese Patent Publication No. 62-22420, and A. Yamada, Y. Jia, M. Konagai, and K. Takahashi, Photochemical Vapor Deposition of Si/Si.sub.l-x Ge.sub.x Strained Layer Superlattices at 250.degree. C., Japanese Journal of Applied Physics, Vol. 27, No. 11, November 1988, pp. L2174-L2176. However, it has not been known to be useful to apply UV radiation to a mist of a liquid precursor in a non-reactive deposition process.